Inhibition of corrosion of magnesium



May 15, 1962 H. K. DE LONG INHIBITION oF coRRosIoN 0F MAGNESIUM Filedsept. 5, 1959 IN VEN TOR. Herben A. De any ATTORNEY v, 3,934,2l PatentedMay 15, 1962 3,034,210 INITION 0F CRRQSION 0F MAGNESIUM Herbert K. DeLong, Clare, Mich., assigner to The Dow Chemical Company, Midland,Mich., a corporation of Delaware Filed Sept. 3, 1959, Ser. No. 837,78) 7Claims. (Cl. 29-458) The invention involves an improved method ofinhibiting corrosion of magnesium metal and magnesium-base alloys whencoupled by or to a less base, Le., more noble metal, and exposed to acorrosive environment. By magnesium base alloys is meant herein an alloycontaining at least about 65 percent by weight magesium metal. Eithermagnesium metal or such an alloy thereof are sometimes referred tohereinafater as magnesium.

When dissimilar metals are placed in contact with each other or arecoupled by a conductor and allowed to remain in such relationship,without specific protection, whether in the atmosphere, the sea, or theearth, at least one of the metals usually undergoes corrosivedeterioration as a result of galvanic action between the dissimilarmetals due to the presence of an electrolyte in the ambient contactingmedium and the difference in the electropotential of the metals.Moisture and other gases suspended in or composing the air provide anelectrolyte in the atmosphere. The Water of the sea and the dis solvedsalts therein make sea water `an exceptionally good electrolyte. Theground waters carrying dissolved minerals therein, particularly in soilsof high organic content and, consequently of an acidic or sour nature,are effective electrolytes.

Metals employed commercially are seldom pure but nearly always containtraces of other metals because the procurement of a pure metal is eitheruneconomical or practically impossible or because an alloy is preferreddue to its superior properties. impure metals and alloys, therefore, aswell as assemblies of manifestly dissimilar metals in contact with oneanother through an electrical conductor, are highly susceptible togalvanic action and the concomitant corrosion of the less noble metal.When dilerent metals are in Contact in a corrosive environment, whetherin the same or in different metal pieces, galvanic currents are to beexpected.

The metal which corrodes during galvanic action is that one which ishigher in the electromotve series of the elements, referred to herein asless noble or more base. The less noble metal is sometimes said to bethe anode of the galvanic pair and as such continues to give upelectrons to the electrolyte and is thereby consumed.

Attempts to lessen corrosion of a metal due to the galvanic etfect ofits being anodic to another to which it is coupled has included the useof sacrificial metal pieces which provide a greater flow of electronsthan a metal to be protected and which are thereby consumed in place ofthe metal sought to be protected. This method is meritorious in a largenumber of situations but entails the cost, installation, and replacementof the sacrificial metal, such metal being relatively expensive. Otherknown methods include the application, to the contacting surfaces of thedissimilar metals exposed to galvanic action, of coatings containingsuch compounds as metal therefore corroded away. A common situation inwhich magnesium is in contact with a more noble metal in a corrosiveenvironment is ,that wherein magnesium pieces are secured by ymeans ofsteel, copper or copper-base alloys, or aluminum rivets or bolts, ormagnesium plates, sheets, or the like are otherwise in Contactwith-steel, aluminum, copper, brass, or bronze plates, studs, conduits,or the like.

A particularly effective known method of protecting .magnesium againstcorrosion by contact with a more noble metal connected thereto by anelectrical conductor is the placement of tin metal or a tin-base alloy,containing at least about 40 percent tin, between the contactingsurfaces of the metal, Such method is described in my copendingapplication S.N. 718,027, led February 27, 1958.

Positioning during fabrication, in an assembly comprising magnesium anda more noble metal, a layer or sheet of tin or tin alloy, or insertingbetween the dissimilar metals after fabrication such tin or tin alloy,is not always practical.

The method described in S.N. 718,027, although representing deliniteprogress in theart, is not adaptable to conditions where it is notpractical to interpose the tin u or tin alloy sheet between thecontacting or abutting surfaces of dissimilar metals. Furthermore, themethod threin described is not adaptable to protecting against corrosionof the less noblemagnesium due to the presence of more noble impuritiescontained within the same metal piece.

There is, accordingly, a need for a more convenient method of protectingmagnesium metal orL a magnesium base alloy coupled toa more noble metalor contaminated by more noble metal impurities or alloyed with a morenoble metal against the corrosive effects induced by the more noblemetal which method includes treatment of either the contacting areas orof the electrolyte-containing medium,

The present invention meets this need. The method of practicingtheinvention is made clear in the ensuing description and isspecifically defined in the appended claims.

The present invention is a method of suppressing corrosion of magnesiummetal or magnesium base alloys due to the galvanic effect produced by amore noble metal coupled thereto in an electrolytic medium whichconsists essentially of maintaining the magnesium metal or its alloy inthe presence of a small but effective amount of a tin compound having asolubility in water of at least 0.05'weight percent at about 20 C.

The drawing illustrates, by a series of lign-res, the

eifectiveness of the invention as demonstratedby EX- ample 3 set forthhereinafter.

The tin compounds suitable for use in the practice of the inventioninclude organic tin compounds, eg., tributyl tin oxide, dimethyl tindichloride, stannous octoate', tributyl tin acetate, and inorganic tincompounds, e.g., sodium stannate, potassium stannate, and stannoustluoride.

The Itin compound may be added directly to the electrolytic medium or itmay be incorporated in a coating which is applied to the surface ofeither the magnesium metal or of the more noble metal or to both metalsurfaces which are in electrical Contact. Any coating material which issatisfactory as a more or less permanent coating to magnesium metal orto the more noble metal may be employed. Illustrative of such coatingsare various paints, eg., pigments dispersed in a suitable vehicle, eg.,zinc chromate, lead sulfate, lead oxide, or iron oxide, or the vehiclealone without the pigment, or a synthetic resin, e.g., a bu'tyral,phenolic, or alkyl type resin, or shellac, a common varnish, enamel, ora rubber or polyvinyl chloride coating in a suitable organic vehicle.

The organic tin compound or inorganic tin compound when added directlyto the electrolytic medium according to the practice of this inventionis employed in an amount of at least about 0.2 weight percent of theelectrolyte. The maximum amount employed is not highly critical but inthe interest of economy not more than 2 percent by weight of theelectrolyte and usually not over 1 percent is recommended. When the tincompound is employed according to the invention by incorporating it intoa coating material, between 1 and 25 percent by weight of the tincompound based on the weight of the coating material is employed andusually between 2 and 15 percent. The thickness of the coating materialis not highly critical and varies with the composition of the material,method of application, and other considerations for determining thethickness of the coating desired.

The suppressing eiTect of the tin compound when used according to theinvention is enhanced when there is added either to the electrolyticmedium or to the coating material, dependent upon the mode of practicingthe invention being employed, a soluble nitrate, e.g., an alkali metalor alkaline earth metal nitrate, illustrative of which are magnesiumnitrate, calcium nitrate, and lithium nitrate. The nitrate is added inan amount between about 0.1 and percent by weight of the electrolyte orcoating composition containing the tin compounds. Lithium nitrate is thepreferred nitrate to employ.

The following examples are illustrative of the practice of theinvention:

Example 1 A sheet of a magnesium alloy designated AZ31BO, comprisingabout 3 percent aluminum, about 1 percent zinc, about from 0.15 to 0.20percent manganese, and balance magnesium was sanded and test panels 1 x2 x 0.025 inch in size were cut therefrom.

Two holes were drilled in a test panel, thus prepared, of a suitablesize to accommodate 'S inch diameter bolts. Threaded, round-headed steelbolts @is inch in diameter and 5% inch long were then inserted into theholes and nuts applied to the threaded end thereof and firmly tightenedso that a good electrical contact was made between the steel of thebolts and the magnesium metal of the panel. 250 milliliters of a 1percent by weight sodium chloride brine was prepared. For comparativepurposes, the 4magnesium panel containing the steel bolts was submergedin the brine at room temperature without further treatment of the brineand retained therein for 24 hours.

The panel was then removed from the brine, Washed, and dried and foundto be severely corroded in the area adjacent to and surrounding thesteel bolts.

An arbitrary scale, ranging in lessening corrosion se- Verity from 0 tol0, was then set up in which the corrosion induced on the magnesiumpanels containing the steel bolts (when thus submerged in 4the brinewithout inhibition) was rated 0 and no corrosion was given a rating of10.

To contrast the corrosive effects of the steel bolts on the panel in theassembly prepared above to such elects, if any, when the invention waspracticed, a second assembly was prepared as above and placed in asecond solution of NaCl brine as above except that 0.5 percent by weightof sodium stannate hydrate, Na2SnO3-H2O, was admixed with the NaCl brineprior to submerging the magnesium panel-steel bolt assembly therein.Sodium stannate is readily water-soluble. The panel containing the boltswas then submerged in the NaCl brine containing the sodium stannate andmaintained therein at room temperature for 24 hours. The panelcontaining the steel bolts was then removed from the brine, washed,dried, and examined. Very little indication of corrosion could bedetected on the panel surface adjacent to the steel bolts. The panel wasgiven a corrosion rating of 9 (10 representing absolutely no corrosion).

Example 2 Example l above was repeated except -that 0.5 percent ofLiNO3-3H2O was dissolved in the NaCl and Na2SnO3. SH2() brine solutionprior to submergence therein of the magnesium test panel containing thesteel bolts. After submergence for 24 hours, the panel was removed,washed, dried, and examined as above. The corrosion of the panel aboutthe steel bolts was even less than that of Example 1. The panel wasgiven a corrosion rating of 9.5.

Example 3 The drawing is illustrative of-this example, wherein amagnesium alloy panel is represented by numeral 1; three 1A bolt holes,first drilled in the panel, by numeral 2.; an anodized coating bynumeral 3; three additional 1A bolt holes made in the panel subsequentto the anodized coating by numeral 4; a resin-bodied coating, with orwithout a protective metallic compound suspended therein, by numeral 5;six 1A" bolts positioned in the six bolt holes and tightened rmlyagainst the panel by numeral 6; a second resin-bodied coating, with orwithout a protective metallic compound therein, by numeral 7; a scrapedaway or bared portion at lthe junction of the magnesium alloy panel andthe steel bolts by numeral 8; the corrosion effects of a salt spray teston the bared magnesium alloy surface, when no tin compound was presentin the resin-bodied polymer coating, by numeral 9; a relativelyuncorroded bared magnesium panel area, when a tin compound was presentin the resinbodied polymer coating in accordance with the invention, bynumeral 10.

An AZ3-1BO panel having a finished sanded size of 4 x 6 x 0.25 inch hada row of three substantially equispaced holes drilled therein toaccommodate three 1A x 1 inch bolts, as Ishown in FIGURE 1. Themagnesium panel containing the three holes was then given a standardpaint base anodizing treatment` designated H.A.E. Anodize according toMilitary Specification MIL- -C-13335. Thereafter, another row of thethree substantially equi-spaced holes, of the same size as the rstthree, were drilled in the anodized panel thereby presenting three holesprotected by anodize treatment and three holes having the interiorthereof unprotected by the anodize treatment as shown in FIGURE 2.

A standard vehicle of the type employed in the preparation of a primerknown as Unichrome AP-lO chromate primer, but in this instance withoutthe zinc chromate or other pigment, was obtained. The body of thevehicle consists of resin and shellac dispersed in an organic vehicle.The resin usually is polyvinyl butyral and i-s present in a weight ratioof about 62 parts to 38 parts of shellac. The suitable organic carrieris usually about a 50:50 volume ratio of toluol and isopropyl alcohol.The vehicle thus described was sprayed onto the panel including theinterior of the six holes and dried. Thereafter six 1A x 1 inch threadeddat hea-ded steel bolts were inserted into the holes, nuts lscrewedthereon, and the nuts tightened to make a r-m contact between the steelheads of the bolts and the magnesium panel as shown in FIGURE 3.Thereafter a second coat of the Unichrome AP-lO primer, again withoutpigment therein, was sprayed onto the magnesium panel containing the sixsteel bolts, as shown in FIGURE 4. The coating was cured by forced airdrying at F. for 30 minutes. After drying, the coated surface of thepanel containing the bolts was scratched through to bare the magnesiumand steel surfaces at their junction, as shown in FIGURE 5. The panelwas then subjected to a 20 percent salt spray fog test in accordancewith -Method 606.1 of Federal Specication TT-P-l41-B for 85 hours. Afterthis period of exposure to the salt spray, the panel was removed,

dried and examined. Both the exposed magnesium surface adjacent to thesteel bolts and the magnesium surface coated with the bodied vehiclecontaining no zinc chromate or other inhibiting materials,` wereseverely c or. roded, as shown in FIGURE 6. Both the unscratched and thebared magnesium surfaces were given a corrosion rating of 0.

For further comparative purposes the above test was repeated in exactlythe same manner except that the Unichrome AP-lO -bodied primer vehicleemployed contained zinc chromate in the ordinary amount used to protectmagnesium surfaces. The panel thus coated was subjected to the salt-spray test described above. Examination of the thus treated magnesiumpanel following removal from the salt spray fog test showed severecorrosion in the area of the magnesium panel adjacent to the steelbolts, similarly as shown in FIGURE 6. The corrosion was of equalseverity about each bolt showing that anodizing the interior of theholes had no benecial effect upon the prevention of corrosion about theheads of the bolts. The corrosion, however, was almost, though not quiteso severe on the thus ltreated panels as in the test above wherein nozinc chromate was present. The corroded panel was given a relativecorrosion rating of 1.

The above test employing the Unichrome AP-l lbodied vehicle withoutpigment was repeated but, in accordance with the practice of theinvention7 there was incorporated into the primer vehicle (whichcontained no zinc chromate) 15 percent by weight of tributyl tin oxide.Tributyl tin oxide has a water-solubility of about 0.71 percent at roomtemperature. The thus treated vehicle was sprayed on the magnesium panelboth prior to insertion of the bolts and following the insertion of thebolts and dried as above described and illustrated in FIGURES lof thedrawing. Thereafter the panel containing the steel bolts was subjectedto the salt spray test as above. The panel was then removed, washed,dried, and examined. Some corrosion was -found onthe magnesium panelsurfaces adjacent to the steel bolts but it was clearly of definitelyless extent than that shown in either of the above comparative tes-ts.FIGURE 7 illustrates the comparative uncorroded area which occurs whenthe protective coating contains a tin compound in accordance with theinvention in contrast to the corroded area which results when theprotective coating does not contain a tin compound in accordance withthe invention. There was no dierence in the degree of corrosion of themagnesium panels about the steel boltswherein the interior of the boltholes had been anodized prior to being coated and wherein the bolt holeshad not been anodized. The panel was given a corrosion rating of 7.

The tests were repeated employing decreasing percents of tributyl tinoxide. Some benecial effect on inhibiting corrosion was observed `downto l percent by weight of the tin compounds based on the weight of thecoating composition. It was found that as much as 25 to 30 percent ofthe tin compound can be used but between 2 and l5 percent isrecommended.

Example 4 Example 3 was repeated except that 15 percent by volume ofstannous octoate instead of the tributyl tin oxide was admixed with theUnichrome AP- clear primer vehicle (no pigment therein) in coating amag-Y Y 6 Example 5 Example 3 was again repeated except that 10 percenti by volume of tributyl tin acetate, instead ofthe 15 percent tincompound there employed was admixed with the Unichrome A13-l0 clearprimer in coating the magnesium alloy panel containing the steel bolts.Tributyl tin acetate has a water solubility of about 0.06 weight percentat room'temperature.

The magnesium alloy panel thus prepared was subjected to the salt sprayfog test as above described. -Examination of the panel containing thebolts showed some corrosion of the panel adjacent to the bolts. ThepanelV was given a corrosion rating of 8 according to the scaledescribed above.

To show that the tin compound must possess a minimum degree ofsolubility in water, Example 1 above was repeated except that 0.1percent of magnesium stannate hydrate, MgSnOaI-IZO, was admixed with a 3percent sodium chloride aqueous solution. Magnesium stannate issubstantially insoluble in water. After 24 hours of submergcnce in thesalt solution, containing the magnesium stannate, the panel was removed,washed, dried, and examined. The examination of the panel which had beenthus submerged, showed severe corrosion in the area of the paneladjacent to and surrounding the place of contact with the bolts. Thepanel was given a corrosionV rating of 0. it is clearly apparent thatunless some of the tin compound goes into solution, it does not inhibitthe corrosion of the less noble metal of a bimetallic couple subjectedto a corrosive environment.

An examination of the examples of the invention and test runs made forcomparative purposes shows that a tin compound, having a watersolubility of at least 0.05 weight percent, when admixed with anelectrolyte in con-i tact with magnesium or its alloy coupled to a morenoble metal, markedly suppresses the corrosion of the magnesium due togalvanic action.

rEhe examination Vof such examples and tests further shows that tincompounds, whose water solubility is at least 0.05 weight percent, whenadmixed with conventional coating compositions accordingto the inventionin an amount of 1 percent by weight or more of the coating compositionand the composition thus prepared applied to that surface of a magnesiumpiece or article which is subsequently brought into contact with themore noble metal, also markedly suppress the corrosion of the magnesiumdue to galvanic action.

The examples and tests also show that the suppression. in accordancewith the method of the invention, of corrosion of magnesium when coupledto or in contact with a more noble metal is far superior to such knownmethods of suppressing as anodizing, applying resin coatings, andapplying such resin coatings containing zinc chromate to the surface or"either the magnesium or the more noble metal which may be brought incontact with each other.

Having described the invention what is claimed and desired to beprotected by Letters Patent is:

1. The method of suppressing corrosion of magnesium metal whenelectrically coupled to a more noble metal in an electrolytic mediumwhich consists of maintaining the magnesium metal in the presence of asmall but etiective amount of a tin compound having a water-solubilityat 20 C. of at least 0.05 weight percent.

2. The method of suppressing corrosion of magnesium metal whenelectrically coupled to a more noble metal in an electrolytic mediumwhich consists of dissolving in said electrolytic medium a tin compoundhaving a water solubility of at least 0.05 weight percent at 20 C.

3. The method of claim 2 wherein the tin compound is present in anamount between 0.2 and 2.0 percent by weight of the electrolytic medium.

4. The method of claim 3 wherein a soluble nitrate is present in saidelectrolytic medium in an amount between 0.1 and 5.0 percent by weightof said electrolytic medium.

5. The method of suppressing the corrosion of an article composed of atleast 60 percent magnesium metal when electrically coupled to a morenoble metal in an electrolytic medium which consists of maintaining saidmagnesium article in the presence of small, but effective amounts ofboth a tin compound selected from the class consisting of tributyl tinacetate, tributyl tin oxide, dimethyl tin dichloride, stannous octoate,sodium stannate, potassium stannate, and stannous fluoride, and a metalnitrate.

6. The method of claim 5 wherein the metal nitrate is lithium nitrate.

7. The method of suppressing corrosion of an article composed of atleast 60 percent magnesium when electrically coupled to a more noblemetal in an electrolytic medium which consists of incorporating into afilm-forming composition a tin compound having a water-solubility atabout 20 C. of at least 0.05 weight percent in an amount of at least lpercent and a soluble metal nitrate References Cited in the file of thispatent UNITED STATES PATENTS 2,240,021 Rutherford Apr. 29, 19412,398,738 Gilbert Apr. 6, 1946 FOREIGN PATENTS 1,062,866 Germany Aug. 6,1959 OTHER REFERENCES Magnesium (Pidgron et al), published by AmericanSociety of Metals (Cleveland, Ohio). (Pages 200, 205,

in an amount of between 0.1 and 5.0 percent, by Weight 20 24@-2440

7. THE METHOD OF SUPPRESSING CORROSION OF AN ARTICLE COMPOSED OF ATLEAST 60 PERCENT MAGNESIUM WHEN ELECTRICALLY COUPLED TO A MORE NOBLEMETAL IN AN ELECTROLYTIC MEDIUM WHICH CONSISTS OF INCORPORATING INTO AFILM-FORMING COMPOSITION A TIN COMPOUND HAVING A WATER-SOLUBILITY ATABOUT 20* C. OF AT LEAST 0.05 WEIGHT PERCENT IN AN AMOUNT OF AT LEAST 1PERCENT AND A SOLUBLE METAL NITRATE IN AN AMOUNT OF BETWEEN 0.1 AND 5.0PERCENT, BY WEIGHT OF THE FILM-FORMING COMPOSITION, APPLYING SAIDCOMPOSITION IN AN UNCURED STATE TO AT LEAST ONE OF THE SURFACES OF THEMAGNESIUM METAL ARTICLE AND THE MORE NOBLE METAL WHICH WILL BE INELECTRICAL CONTACT WHEN COUPLED TOGETHER, COUPLING THE MAGNESIUM ANDMORE NOBLE METAL, AND CURING SAID FILM IN SITU.